Illumination arrangement for recording and/or reproduction in color

ABSTRACT

An illumination arrangement for use in connection with picture recording or reproduction in color. The device automatically compensates for changes in the spectral composition of the light at various light intensities. In one embodiment, the device includes a first lamp and a second, smaller, lamp, the lamps being coupled such that a change in the color temperature in one direction of a first lamp is compensated for by a change in the color temperature of a second lamp, the change in the color temperature of the second lamp being such in its direction and amount, to compensate for the change in color temperature of the first lamp.

United States Patent J armar 54] ILLUMINATION ARRANGEMENT FOR RECORDING AND/0R REPRODUCTION Feb. 22, 1972 FOREIGN PATENTS OR APPLICATIONS 506,727 5/1939 Great Britain ..355/88 IN COLOR 253,968 12/1948 Switzerland ..355/37 [72] inventor: Sven O. Jamar, Huddinge, Sweden OTHER PUBLICATIONS Assignee! sYel'iges Radio Akfiebolag, Stockholm, Brightness Variation Without change of Colour-Tempera- Swede" ture," 7 European Broadcasting Union Review, Part A- O Technical, April NO. 1 l4, and [21] 'AppLNo.: 863,553 Primary Examine rRaymond F. Hossfeld Attorney-Larson, Taylor and Hinds [30] Foreign Application Priority Data [57] ABSTRACT Oct- 9, Sweden illumination arrangement for use in connection pic, ture recording or reproduction in color. The device automati- [52] US. Cl ..315/298, 315/296, 315/297, cally compensates for changes in the spectral composition of 355/35, 355/37 the light at various light intensities. in one embodiment, the [51] Int. Cl. ..H05b 41/39 device includes a first lamp and a second, smaller, lamp, the [58] Field of Search ..315/152, 153, 154, 296, 297, lamps being coupled such that a change in the color tempera- 315/293; 355/3 5 37 70 33 ture in one direction of a first lamp is compensated for by a change in the color temperature of a second lamp, the change [56] References Cited in the color temperature of the second lamp being such in its direction and amount, to compensate for the change in color UNITED STATES PATENTS temperature of the first lamp.

2,438,219 3/1948 Johnston ..355/37 18 Claims, 8 Drawing Figures -VOLTAGE LIGHT FLUX 20 l l h H T l2 16:45 e

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' PAIENTEBreaza I972 SHEET 3 OF 6 AUXILIARY LAMP LIGHT MAIN LAMP LIGHT NORMAL WAVE LENGTH ILLUMINATION ARRANGEMENT FOR RECORDING AND/R REPRODUCTION IN COLOR In color photography and color recording for reproduction by means of television and in many similar cases, a control of the light intensity must take place successively. This control, as a rule, takes place by changing the voltage fed to an incandescent lamp. However, it is well known that at a change of this voltage the color distribution in the spectrum of said incandescent lamp is also changed, usually in such a way that an essentially stronger proportion of red light will be created, the lower the voltage is chosen. This variation in color distribution within the used light, of course, may be compensated for by a change of the sensitivity of the recording apparatus for different colors. As a rule, this simple method is for practical reasons not available, because one cannot repeatedly change film in a camera so that the used film will be of the correct chemically optical composition, and also one cannot work with suitable adjustment filters, nor can one during a recording for television change the amplification of the three basic color amplifiers, because the changes in light intensity which occur will usually happen too often, too quickly, and too unexpectedly.

Therefore, there is a strongly felt need for an illumination arrangement, which is created in such a way that it will automatically at all existing light intensities give one single spectrotechnical composition of the light.

In this connection it will be helpful to discuss the term color temperature," which is rather difficult to define but which is often used in the art. A typical temperature of the incandescent wire in a lamp will be about 3,000 K, At this temperature the lamp will give off a light, which has a predetermined spectral composition, which is often understood to be completely white, although this is certainly far from correct. If one decreases the voltage, then the temperature of the incandescent wire will also decrease, and simultaneously the color of the incandescent wire is changed, or, as one could also express it, the gravity point of the illumination given off is displaced in a direction towards red. One then says, that the lamp has got a lower color temperature, which should, as a matter of fact, be understood the way that the lamp has got a lower temperature, which is combined with such a change of color, which regularly will enter at a decrease of the temperature. In a corresponding way, a lamp may of course also be given a higher color temperature, whereby the gravity point within the color spectrum will be displaced in direction into the blue or violet.

In the vicinity of the normally existing color temperatures one counts with a tolerance amount of about iIOO C., which means, that no disturbing color error setting will exist in the reception, if the temperature should vary upwardly or downwardly by the said amount of tolerance of 100 C. This tolerance value, of course, will not have validity at lower color temperatures, which differ very essentially from the normal color temperature.

The present invention now refers to an illumination arrangement, by means of which one may control the color intensity and the illumination by changing the voltage without thereby changing the color temperature to a further extent than a change within the allowed limits of tolerance.

According to the invention, at least two lamps are arranged in such a coupling that a change of the color temperature in one direction of one, essentially greater, lamp will be followed by a change of the color temperature of the other, essentially smaller, lamp, the latter change being so adapted as to its direction and amount, that it causes a compensation of the change of color temperature of the first-mentioned lamp.

The first-mentioned lamp will in the following be called the main lamp", whereas the latter lamp will be called the compensation lamp. However, it is obvious that the invention is not limited to one single main lamp and one single compensation lamp, but that complete batteries of lamps of each kind may exist, and that the lamps in one and the same battery may not even necessarily be subjected to the same color temperature change. On the other hand, one can, of course, also simplify the form of execution in the way that one will use one single lamp unit which, in a way known per se, for lamp," known from the automobile head lamp, contains two incandescent wires. Such modifications, of Course, will also fall within the scope of the present invention.

The light fluxes from the two lamps should be combined into one single light beam. By suitable adaption between the light intensities of the two partial light beams one can then change the color of each partial light beam, so that in spite of the variations in the light intensity of the total light flux, its color, or as one used to say, its color temperature, will remain constant. By this is meant that the color composition will remain constant at a state corresponding to the normal color temperature of the lamps, although the color temperature of the one lamp has increased and the color temperature of the other lamp has decreased. As a matter of fact, one is here placed before an equilibrium relation with the desired color temperature as a reference point. A weak light flux in a greater temperature difference from the desired temperature thus may compensate as far as regards the color composition a smaller temperature difference in the other direction of a stronger light flux and vice versa.

The invention will be further described below in connection with the attached drawings, which show different embodiments of the invention. However, it is understood that the in vention is not limited to the arrangements thus shown in the drawings and further described below, but that other modifications may occur within the frame of the invention.

In the drawings:

FIG. I shows a normal diagram showing the variation of the color temperature as well as of the light flux with a change of the voltage of an incandescent lamp.

FIG. 2 shows a family of curves, indicating the spectral energy distribution in wavelength with the incandescent temperature as an arbitrary magnitude, measured as a function of the relative power, and

FIG. 3 shows one of the curves of the curve family according to FIG. 2 as well as the corresponding curve for a light flux composed of the flux from two lamps according to the invention, as the sum of the two partial light fluxes also shown in FIG. 3.

FIG. 4 shows a simple arrangement, by means of which one may with allowable approximation achieve a substantially constant color temperature independently of the variation in light flux, whereas FIGS. 5 and 6 show a couple of different arrangements, more advanced for the same purpose, in block diagram.

FIG. 7 shows a complete diagram of a system of the last mentioned kind, in which, however, such means which are already commonly known have been shown in block diagram for simplification of the description.

FIG. 8 finally, shows an auxiliary diagram, which may be used for the calculation of the numerical values of the magnitudes contained in the diagram according to FIG. 7.

The diagram according to FIG. 1 refers to a known lamp having an incandescent wire, the normal temperature ofwhich was 3,000 K. at normal voltage. The voltage scale runs along the horizontal axis and is divided into percent of the normal voltage. Also the vertical axis is divided into percent of the normal value, viz regarding the curve A of the light flux and regarding the curve B of the color temperature. It is seen, that the one curve is concave upwardly, whereas the other one is strongly convex upwardly.

It is also seen from FIG. 1, that at a decrease of the voltage from its normal value, the light flux according to the curve A will sink very strongly, whereas, on the other side, the color temperature will sink only rather slowly in the beginning, later on however still quicker. When the voltage has been decreased to about 20 percent, the light flux, by which is meant the flux of the light, observable by the human eye, is mainly equal to zero. Simultaneously the color temperature has decreased to about 53 percent. Below this value, the radiation from the incandescent lamp will thus mostly have infrared been indicated by 100 percent corresponds to 3,000 K., and

that the tolerance range was put at 1:100 corresponding to 3 .3 percent.

Detailed statements about the displacement of the different light wave lengths and the relative power represented by them at different temperatures of the incandescent wire will be seen from FIG. 2. The relative power thereby is plottedalong the vertical axis, whereby the normal value is assumed to be at 1.0. The curves, thus, show its variation up to the double of the normal value. The simultaneously occuring wave length displacement is read along the horizontal axis.

The curve shown in FIG. 2, valid for a color temperature of 3,000 K., is reproduced in FIG. 3 by a dotted line. It is now assumed that a main lamp, The color temperature of which has been decreased to 2,100 K. by lowering its voltage and light flux, should get a compensation for its loss of color temperature by means of an auxiliary lamp, having the color temperature of 3,200 K. By the equilibrium relation already mentioned, which, of course, is approximate one may get the result that the lamp the color temperature of which is 2,l00 K. should give about l8 percent of the total light flux whereas the other lamp should give about 82 percent. Curve D in FIG. 3 now has a surface integral, which is 18 percent of the surface integral of the curve in FIG. 2 corresponding to the temperature 2,l00 K., whereas the curve C has a surface integral representing 82 percent of the surface integral of the curve, which applies in FIG. 2 to 3,200" K. Adding now the two curves C and D, you will get the curve E. This rather exceedingly well agrees with the normal color temperature curve, which is thus reproduced by the dotted curve F. Assuming that the curve F as to its contents oflight power represents the maximum light of the lamp combination, then, of course, the real value of the curve E will be essentially less than the curve F. The two curves, however, only indicate relative values, which means that their surface integrals have been made equal in order that one should be capable of comparing them and of seeing if they have the same or essentially different color temperatures.

A simple control device which can be used for a rather coarse manual control of the two lamps is shown in FIG. 4. The main lamp is there indicated by 10 and the compensation lamp by 11. Both of these lamps are connected to the power mains 12 through one resistor 13 or 14, resp. These two resistors are variable and are controlled in common by means of one single control means, indicated by the dotted line 15. The resistor 13 is arranged as a conventional rheostat, so that when its slider 16 is moved to the right in the drawing, then the resistance will increase and consequently the voltage remaining for the incandescent wire of the lamp 10 will decrease. The main conductor, however, is not connected to the terminal of the resistor 14 of the compensation lamp 11 but to an intermediate point 17. When the slider 18 of the resistor 14 is exactly opposite to this connection point 17, the compensation lamp will therefore get full voltage, but if one moves the control means 15 in the one direction or in the other one, resistance will be connected in series with the compensation lamp, and its current will decrease. The purpose of this will be further explained below in connection with FIG. 8.

When the slider 16 is moved over the rheostat 13 the voltage is changed, for instance according to the curve 19 (FIG. 4), and simultaneously the light flux is changed according to the curve 20 (FIG. 4). The corresponding curves for the function of the rheostat 14 are formed, as far asregards the voltage by the curve 21 and as far as regards the light flux by the curve 22. The curve 22 is calculated in a way. which will be evident from the following, so that it will cause as far as possible compensation for the change of color temperature in the main lamp 10.

When winding the resistor screw of the rheostat 14 one should take care, as far as needed, the winding turns within different parts should be placed at unequally distances from each other. By this step one will get an opportunity of influencing the run of the curves 2l and 22. In the ideal case, as a matter of fact, the curve 21 should not be composed by two straight lines, as it has been shown in FIG. 4 for simplification of the description. Even with a voltage variation curve, which is composed in this way by two straight line parts, however, one will get a rather well acceptable result.

The arrangement now described functions mainly in the following way: At full light the main lamp 10 will give a light flux corresponding to what was indicated in FIG. 2 by the value 10. Its color temperature then is equal to the working or normal color temperature, also indicated by the value 1.0. In this position, the compensation lamp 11 should be tuned to the working color temperature, and its light flux should suitably be in the order of magnitude about 0.1. If now the voltage of the main lamp 10 is decreased, then the light flux from this lamp will also decrease; simultaneously, however, also its color temperature is changed. For compensation of the decrease of the color temperature the compensation lamp 11 must get a higher voltage. Thereby the color temperature of this lamp does not only increase, but also its light flux. Due to the compensation lamp being rather small as compared with the main lamp, however, the increase of light will not be as great as the decrease oflight flux from the main lamp, and the total light flux consequently becomes smaller. When the voltage of the main lamp 10 has been decreased such, that its color temperature has decreased to 0.9, the moment from the main lamp is the highest one. Consequently one will have to give the highest voltage to the compensation lamp in this state, corresponding to the position 23 on its rheostat 14. At continuous decrease of the voltage of the main lamp 10, the voltage of the compensation lamp must also be decreased for maintaining correct color composition, initially a little stronger but thereafter successively weaker. Finally, the main lamp becomes completely dark, whereas the compensation lamp will still show a light radiation having a voltage and a color temperature which are identical withthe one which existed at the beginning of this operation.

In the starting position, the composed light flux has in this case an intensity of l.0+0.l=l.l, and in the final position, when only the compensation lamp is carrier of visible light, the intensity was 0.1, which means that the total light intensity has decreased in the proportion of l/l l of the maximum light flux. This is under normal circumstances completely sufficient for the lighting changes which may be required. During all of this variation of the light flux, the color composition or the color temperature has thus been kept constant, and a picture having correct colors is thus obtained.

FIG. 5 shows a further development of the basic idea of the arrangement according to the present invention as shown by means of FIG. 4. In this arrangement voltage regulators have been placed between the mains 26 on the one hand and the main lamp 27 as well as the compensation lamp 28 on the other hand. These voltage regulators in the simplest case, comprise controlled rectifiers, which are controlled in much the same way as the device according to FIG. 4, and therefore the different details have been provided with the same reference numerals. For improving the control power of the rheostat 14, however, a fixed resistor 29 has been connected in series with this rheostat, whereby it is possible to connect one terminal of the fixed resistor 29 to one main terminal and the other terminal to the two ends of the rheostat 14, whereby this will be subjected to a fixed current, which does not vary with the load of the compensation lamp 28, and whereby only the control voltage, without any substantial consumption of current, is derived from the slider 17 in the voltage regulator 25. This means a more sure and a more reliable control method. Of course, as the rheostat 13 of the main lamp 27 has also been replaced by a voltage divider coupled resistor, also this will be subjected to a constant current, and only the control voltage, without any substantial consumption of current, will be derived to the voltage regulator 24.

This arrangement may be further improved by causing the control voltage of the main lamp to determine in cascade coupling the control voltage of the compensation lamp 11, see FIG. 6. Also in this case the control voltage is obtained between the conductors 12, but the voltage required for feeding the lamp is obtained from the main 26. Two voltage regulators are provided as was the case according to FIG. 5, but in this case only the voltage regulator 24 is controlled by means of the potentiometer 13 with the slider 16. The output voltage from the voltage regulator 24 is in this case conducted over the conductors 30 to a control unit of such character, that it will revert the control voltage between the conductors 30 to a control voltage of another value between a pair of conductors 32, running to the voltage regulator 25. The control unit 31 is of such a character that the output voltage from the voltage regulator 24 will be reverted according to a nonlinear function in agreement with curve 33, the horizontal axis of which represents the input voltage over the conductors 30, and the vertical axis of which represents the output voltage over the conductors 32. Consequently, lamp 10 will be provided with a variable voltage, the value of which is determined by the voltage regulator 24 in agreement with the adjustment of the potentiometer 13, whereas simultaneously but in a controlled nonlinear relation, the lamp 11 is provided with a voltage, suitable for the compensation.

It should be explained in this connection why curve 33 in FIG. 6 or the curve 22 in FIG. 4, resp., has the shape, seen in the figures concerned. When the voltage of the lamp 10 is decreased, the color composition of the light produced by lamp 10 changes to a state, containing more red and less blue and violet. For compensation thereof the voltage of the compensation lamp should be increased, so that it shows a color, containing proportionally less red and more blue and violet. Initially one therefore increases the voltage of the compensation lamp, simultaneously as one decreases the the voltage of the main lamp. At a given decrease of the lamp, this will perhaps still show a stronger participation of red light, but simultaneously the light intensity of the main lamp has decreased so strongly, that the composition lamp will give off a continuously increasingly proportion of the total light flux. If one should thereafter continueto increase the voltage of the compensation lamp and to decrease the voltage of the main lamp, then the redish light of the main lamp would play a completely subordinated roll in comparison with the successively increased light intensity of the compensation lamp, and the total light flux would be overloaded by blue and violet light from the compensation lamp. It is therefore necessary, at a given value of the voltage relation between the two lamps, to decrease the voltage of the compensation lamp, so that this will have regained its normal color temperature at the moment, that the main lamp becomes completely dark, in other words the voltage of the compensation lamp should at this moment again have been decreased to its initial value. The light from the compensation lamp will therefore be the only remaining light.

There is no difficulty to calculate or to, graphically construct a curve according to which the voltage of the compensation lamp should vary in the above-mentioned way, once one knows the curves for the mutual dependence of the voltage and the color temperature as well as the light flux existing at each individual voltage from each of the lamps. The construction of the curve 22, FIG. 4, or the curve 33, FIG. 6, therefore is a purely graphical construction step, which every man skilled in the art may execute, once he has knowledge about the principle of the present invention. FIG. 7 shows a complete wiring diagram of a system according to the invention, which principally agrees with the system shown in block diagram in FIG. 6. The main for operation of the lamps is here indicated by 34. This main is an alternating current main of voltage of the main constant voltage. For each of the two lamps l0 and 11 there is a pair of controlled rectifiers 35 and 36 as well as 58 and 59, which are connected in the traditional way in pairs in parallel with each other but in opposite directions. One of these rectifier pairs is controlled by means ofthe control unit 54, and the other rectifier pair is controlled by means of the control unit 60. Such control units are priorly known for other purposes. They work in such a way, that a variation of the adjustment on the input sides of the control units causes a variation of the time displacement of ignition pulses, which are fed over the conductors 55 and 56 to the controlled rectifiers 35 and 36 or by means of corresponding conductors are fed to the controlled rectifiers 58 and 59. The control procedure manifests itselfin the present case in a direct voltage, which is fed to the control unit 54 or 60, resp. over the conductor 53 or 63, resp.. controlling the magnitude of the output voltage obtained after the rectifiers 35 and 36 or 58 and 59, resp. Between the controlled pairs of rectifiers 35 and 36 or 58 and 59, resp., on the one hand, and the lamp 10 or 11, on the other hand, there is an equalization filter comprising a series inductance 38 with parallel condenser 39 and a series inductance 61 with parallel condenser 62, resp. Thereby, the path of current to the two lamps 10 and 11 is described. It will now be described, how the voltage to the two lamps l0 and 11 may be controlled.

The potentiometer 50 is connected to the direct voltage between the terminals 51, which is exclusively used for a control voltage. It is without any decisive importance to this invention, how this control voltage is created, but it may suitably be created by rectification of the voltage in the main 34 after suitable down-transformation. The slider 52 on the potentiometer 50 is connected to the conductor 53, and this conductor runs to the control unit 54 and in this way controls the voltage of the lamp 10.

From the conductors 37, between which there is lamp voltage, also voltage is derived to a transformer, the primary winding 40 of which being magnetically coupled to two secondary windings 41 and 42 insulated from each other. Each secondary winding is connected to its individual full-wave rectifier, for instance as shown in the drawing, a Graetz-bridge 43 or 44, resp. The minus terminal of the bridge 43 is connected to the plus terminal ofthe bridge 44, whereas the plus terminal of the bridge 43 and the minus terminal of the bridge 44 are connected to each other through a couple of equalization condensers 45 and 46, so that negative voltage is obtained at the conductor 49, so that the conductor 48 will function as ground, and so that positive voltage is obtained at the conductor 47. These voltages should be used for controlling the voltage to the lamp 11, but as they are directly proportional to the voltage across the lamp 10, these voltages must first be varied, so that the voltage feeding the lamp 11 will be adapted to the form ofthe curve 22 in FIG. 4 or the curve 33 in FIG. 6, resp., which are determined in advance with knowledge about the properties of the lamps l0 and 11.

The control unit 60 in this case is controlled by the voltage difference between the conductor 48, on the one hand, and a conductor 63, on the other hand. The conductor 63 is fed from an amplifier 64, the one input side of which being formed by the conductor 48 and the other input side being formed by the conductor 65. In the conductor 65, a protection resistor 66 is connected, and the other terminal of this resistor is connected through the conductor 83 to the contact point 84. Between the point 84 and the conductor 48, a further resistor 86 is interconnected, and the current through this resistor consequently determines the voltage which is fed to the one input circuit ofthe amplifier 64. In addition to the current thus running to the conductor of the amplifier 64. In addition to the current thus running to the conductor 48 from the direct current terminals 51 through resistor 85 and the resistor 86, a component of current will also run from the condenser 45 through the resistors 68 and 67 and the point 84, and this component will further tend to run through the resistor 86 to the conductor 48. The second component of current runs in the opposite direction through the conductor 48, the resistor 86,

the resistor 82 and the conductor 81 and the Zener-diode unit 80 to the condenser 46, provided that the path through the Zener-diode unit 80 interconnected in the arrangement is open. Between the conductors 69 and 48 a further battery of Zener-diodes 70-74 is arranged, tuned for different ignition voltages. Each of the Zener-diodes 70-74 is connected in series with a resistor 75-79. Both of the Zener-diode batteries thus are composed by a number of Zener-diodes, connected mutually in parallel but tuned for different ignition voltages, each having its individual series resistor.

The function of the arrangement as described up to this point is as follows:

When adjusting the potentiometer 50 to a lower voltage for the lamp 10, the voltage across the condensers45 and 46 also decreases. At full voltage on the lamp 10 the voltages across the condensers 45 and 46 is maximum, and all of the Zenerdiodesare then conductive. The current, which the voltage across the condenser 45 tries to press through resistor 86, then is equal to the current which the voltage across the condenser 46 tries to press through the same resistor, although in the op posite direction. These two components of current then compensate each other, and only the current from the terminals 51 will flow through the resistor 86. When the voltages across the condensers 45 and 46 decrease due to a readjustment of the potentiometer 50, however one Zener-diode after the other in 80 is extinguished. As a consequence thereof, the component of current which the voltage across the condenser 46 tends to press through the resistor 86 quickly decreases. The second component of current from the condenser 45, on the other hand, does not decrease as quickly, because initially the resistor in this circuit is not changed. A continuously increasing current will therefore flow through the resistor 86 from the point 84 in the direction towards the conductor 48. Thus, the effect will be achieved that a decreasing voltage to the lamp 10 will cause an increasing voltage over the resistor 86. When the last Zener-diode in the aggregate 80 has been extinguished, there is a maximum of current through the resistor 86. As the voltage drop over the resistor 86 by means of the amplifier 64 influences the control unit 60, the voltage on the lamp 11 has also increased to its maximum value.

At continuous decrease of voltage on the lamp 10 with the decrease of voltage on the condensers 45 and 46 following therefrom, a second Zener-diode unit will get into operation. One diode after the other one will be nonconductive, which causes, the voltage on the conductor 69 and therefore also across the resistor 86 to decrease less than the voltage on the condenser 45 and thus also less than the voltage on the lamp 10. When the voltage on the lamp 10 has decreased to zero, only the current from the terminals 51 will flow through the resistor 86 that current is the same value as the one which existed at the beginning of the decrease of the light intensity. The lamp 11 therefore has in this position regained the'same voltage which it has when the lamp 10 had maximum voltage.

FIG. 8, finally, indicates method for determining the curve according to which the voltage on the compensation lamp should vary, and for guidance in the dimensioning of the Zener-diodes as well as of their resistances. The diagram is made in a so-called diagonale diagram. Thus, there are four quadrants having mutually different tasks. The quadrant above the horizontal axis and at left of the vertical axis contains a curve G, which is plotted according to the properties of the main lamp 10. The horizontal axis indicates the relative color temperature of said lamp, whereas the vertical axis indicates the relative light flux of the main lamp. In the quadrant situated at right of the vertical axis and below the horizontal axis, the light fiux of the auxiliary lamp is indicated by the curve I-I along the vertical axis in a direction downwardly, said light flux being measured in relation to the maximum value of the light flux of the main lamp, and the horizontal axis forms a direct continuation of the scale situated at left of the vertical axis. The curve to be determined is situated in the quadrant, which is positioned under the horizontal axis but at the left of the vertical axis. This curve I thus forms the locus of the light flux of the auxiliary lamp in relation to the color temperature of the main lamp, plotted in direction downwardly along the vertical axis. Finally, a pure auxiliary curve, which is required for the construction, is placed in the quadrant situated above horizontal axis and at the right of the vertical axis. This curve is indicated by J.

Given curves thus are G and H. The sought curve is l. The curve I only is an auxiliary curve without other importance for the invention than the one which will be evident from the fol lowing description of the plotting construction.

In this plotting construction one starts from a given, deliberately chosen point on the curve G, for instance the point a. A horizontal lineb is drawn from this point to a point t on the auxiliary curve I, which is assumed by guessing, and from this point a vertical line is drawn downwardly, until it hits the curve H in the point d. One now draws a vertical line e from the point a, also a horizontal line f from the point d. If the point 0 has been correctly chosen, then the crossing point g between the lines e andfshould be situated on the curve I. The check that the construction is correct is obtained by drawing a so-called checking diagonale h through the points 0 and g. If thisruns through the origin, then one has accidentally chosen the point e correctly, but normally one cannot count on success the first time, but one has to use a plotting method, by which one will successively get closer to the correct points. After one has in this way determined a first point on the soughtcurve I one will have to make the same plotting construction starting from another point on the curve G, and successively one will then get the locus for allof the points g obtained, which will together form the sought curve I. Guided by this curve one may thereafter construct the parts of the arrangement, which should indicate the correct voltage to the compensation lamp dependent upon the voltage of the main lamp, so that the combined light flux will get constant color temperature.

I claim:

1. Illumination apparatus to be used in connection with picture recording or reproduction in color comprising a first lamp, a second lamp smaller than said first lamp, means for varying the color temperature of said first lamp, means for varying the color temperature of said second lamp, and coupling means for said color temperature varying means such that a change in the color temperature varying means such that a change in the color temperature of said first lamp causes a change in the color temperature of said second lamp, the latter change being sufficient indirection and amount to compensate for the former change.

2. Illumination apparatus according to claim 1 wherein the normal operating color temperature of said first lamp is substantially the same as that of said second lamp.

3. Illumination apparatus according to claim 1 wherein said means for varying the color temperature of said first lamp comprises means for changing voltage to change the light intensity thereof, wherein said means for varying the color temperature of said second lamp comprises means for changing voltages to change the light intensity thereof; said coupling means being arranged to increase the voltage to said second I lamp when the voltage to said first lamp is decreased from maximum until a point of at least approximate equilibrium between the color temperatures of these lamps is reached, and to thereafter decrease the voltage to said second lamp on further decrease in voltage to said first lamp.

4. Illumination apparatus according to claim 1 wherein said means for varying the color temperature of said first lamp comprises means for varying the voltage to said first lamp comprising a first variable resistor and said means for varying the color temperature of said second lamp comprises means for varying the voltage to said second lamp comprising a second variable resistor.

5. Illumination apparatus according to claim 4 wherein said first and second resistors each comprise slide means for varying the resistance value thereof, and wherein said coupling means comprises a member linking said slides.

said first and second lamps is substantially constant over the range of resistance values of said resistors.

7. Illumination apparatus according to claim 4 wherein said means for changing voltage of said first and second lamps each further comprises a voltage regulator electrically connected to said variable resistor and to said lamp.

8. Illumination apparatus according to claim 7 wherein the means for changing voltage of said second lamp further comprises a fixed connected in series with said variable resistor.

9. Illumination apparatus according to claim 3 wherein said means for changing the voltage to said first lamp comprises a first variable resistor, and said means for changing the voltage to said second lamp comprises a control unit responsive to the output voltage of said first variable resistor to produce a con trol unit output voltage fed to said second lamp, the latter outputvoltage compensating for the color temperatures of said first lamp determined by the setting of said first variable resistor. i 10. Illumination apparatus according to claim 9 wherein said means for changing the voltage to said first lamp further includes a voltage regulator controlled by the output voltage of said variable resistor, and wherein said means for changing the voltage of said second lamp further includes a voltage regulator controlled by the output voltage of said control unit.

11. Illumination apparatus according to claim 9 wherein said control unit is supplied with direct current from a first voltage supply and wherein said first and second lamps are each supplied with alternating current from-a second voltage supply.

12. Illumination apparatus according to claim 10 wherein each of said voltage regulators comprises two controlled rectifiers connected in parallel in the line supplying current to its respective lamp, said rectifiers being connected in electrically opposite directions.

13. Illumination apparatus according to claim 12 wherein said means for changing the voltage to said second lamp comprises a transformer, the primary winding of which is connected into the conductor supplying current from said voltage regulator to said first lamp, the secondary winding thereof being connected to a rectifier for feeding rectified voltage to said control unit.

14; Illumination apparatus according to claim 13 wherein the voltage from said secondary winding is connected to said control unit through a plurality of Zener rectifiers graded by means of ignition voltage resistors, the rectifier grading providing the correct characteristics for compensation of a change in the color temperature of said first lamp.

15. Illumination apparatus according to claim 14 wherein said transformer includes two secondary windings each being connected to one set of Zener-rectifiers along with control voltage controlled resistors one of said set of Zener rectifiers being intended to cause a rise of voltage to said second lamp required in the beginning of the control range, whereas the other set of Zener rectifiers is intended to cause the decrease of the voltage to said second lamp required at the end of the controlled range, said two sets of graded Zener rectifiers being for this purpose connected in directions electrically opposite to each other.

16. Illumination apparatus according to claim 15 further comprising an amplifier connected in the conduit between the Zener rectifiers and the control unit for said second lamp.

l7. Illumination apparatus according to claim 13 wherein said rectifier comprises a double wave rectifier.

18. Illumination apparatus according to claim 17 wherein said rectifier comprises a Graetz-bridges. 

1. Illumination apparatus to be used in connection with picture recording or reproduction in color comprising a first lamp, a second lamp smaller than said first lamp, means for varying the color temperature of said first lamp, means for varying the color temperature of said second lamp, and coupling means for said color temperature varying means such that a change in the color temperature varying means such that a change in the color temperature of said first lamp causes a change in the color temperature of said second lamp, the latter change being sufficient indirection and amount to compensate for the former change.
 2. Illumination apparatus according to claim 1 wherein the normal operating color temperature of said first lamp is substantially the same as that of said second lamp.
 3. Illumination apparatus according to claim 1 wherein said means for varying the color temperature of said first lamp comprises means for changing voltage to change the light intensity thereof, wherein said means for varying the color temperature of said second lamp comprises means for changing voltages to change the light intensity thereof, said coupling means being arranged to increase the voltage to said second lamp when the voltage to said first lamp is decreased from maximum until a point of at least approximate equilibrium between the color temperatures of these lamps is reached, and to thereafter decrease the voltage to said second lamp on further decrease in voltage to said first lamp.
 4. Illumination apparatus according to claim 1 wherein said means for varying the color temperature of said first lamp comprises means for varying the voltage to said first lamp comprising a first variable resistor and said means for varying the color temperature of said second lamp comprises means for varying the voltage to said second lamp Comprising a second variable resistor.
 5. Illumination apparatus according to claim 4 wherein said first and second resistors each comprise slide means for varying the resistance value thereof, and wherein said coupling means comprises a member linking said slides.
 6. Illumination apparatus according to claim 4 wherein at least one resistor comprises a wound resistance wire with a variable winding spacing, the spacing of the windings being such that the color temperature of the combined light from said first and second lamps is substantially constant over the range of resistance values of said resistors.
 7. Illumination apparatus according to claim 4 wherein said means for changing voltage of said first and second lamps each further comprises a voltage regulator electrically connected to said variable resistor and to said lamp.
 8. Illumination apparatus according to claim 7 wherein the means for changing voltage of said second lamp further comprises a fixed connected in series with said variable resistor.
 9. Illumination apparatus according to claim 3 wherein said means for changing the voltage to said first lamp comprises a first variable resistor, and said means for changing the voltage to said second lamp comprises a control unit responsive to the output voltage of said first variable resistor to produce a control unit output voltage fed to said second lamp, the latter output voltage compensating for the color temperatures of said first lamp determined by the setting of said first variable resistor.
 10. Illumination apparatus according to claim 9 wherein said means for changing the voltage to said first lamp further includes a voltage regulator controlled by the output voltage of said variable resistor, and wherein said means for changing the voltage of said second lamp further includes a voltage regulator controlled by the output voltage of said control unit.
 11. Illumination apparatus according to claim 9 wherein said control unit is supplied with direct current from a first voltage supply and wherein said first and second lamps are each supplied with alternating current from a second voltage supply.
 12. Illumination apparatus according to claim 10 wherein each of said voltage regulators comprises two controlled rectifiers connected in parallel in the line supplying current to its respective lamp, said rectifiers being connected in electrically opposite directions.
 13. Illumination apparatus according to claim 12 wherein said means for changing the voltage to said second lamp comprises a transformer, the primary winding of which is connected into the conductor supplying current from said voltage regulator to said first lamp, the secondary winding thereof being connected to a rectifier for feeding rectified voltage to said control unit.
 14. Illumination apparatus according to claim 13 wherein the voltage from said secondary winding is connected to said control unit through a plurality of Zener rectifiers graded by means of ignition voltage resistors, the rectifier grading providing the correct characteristics for compensation of a change in the color temperature of said first lamp.
 15. Illumination apparatus according to claim 14 wherein said transformer includes two secondary windings each being connected to one set of Zener-rectifiers along with control voltage controlled resistors one of said set of Zener rectifiers being intended to cause a rise of voltage to said second lamp required in the beginning of the control range, whereas the other set of Zener rectifiers is intended to cause the decrease of the voltage to said second lamp required at the end of the controlled range, said two sets of graded Zener rectifiers being for this purpose connected in directions electrically opposite to each other.
 16. Illumination apparatus according to claim 15 further comprising an amplifier connected in the conduit between the Zener rectifiers and the control unit for said second lamp.
 17. Illumination apparatus according to claiM 13 wherein said rectifier comprises a double wave rectifier.
 18. Illumination apparatus according to claim 17 wherein said rectifier comprises a Graetz-bridges. 